The present invention relates to displays visible to a user of an environmental control device, especially a display on a programmable thermostat.
Dot-matrix type LCD's, which have display elements arranged in rows and columns, are used in great numbers. These LCD's available at present are classified into three types. The first is a positive type wherein some of the display elements become opaque when driven, thus defining characters, symbols, and graphic characters. The second is a negative type wherein some of the display elements become opaque when driven, whereby the remaining display elements define characters, symbols, and graphic characters. The third is a positive-negative type, which functions as a positive type when operated in the first operating mode and as a negative type when operated in the second mode. With most LCD's available now, it is possible to change the image contrast, that is, the ratio in brightness of the data items displayed to the background of the screen.
Dot-matrix type LCD's are common as user-visible displays on programmable thermostats for viewing of alphanumeric and iconic information. The displayed information relates to environmental conditions, HVAC equipment operation or non-operation, operational modes of the thermostat, and the like. Such displays are commonly lighted from behind or from the side. The backlighting from this planar element shines through non-opaque display elements of the LCD display, although the backlighting is blocked by display elements of the LCD display that are partially or completely opaque. Prior art thermostats have consistently been programmed so that data items are shown on the LCD as partially or completely opaque display elements.